WO2015159965A1

WO2015159965A1 - Hot-rolled steel sheet having good cold workability and excellent hardness after working

Info

Publication number: WO2015159965A1
Application number: PCT/JP2015/061767
Authority: WO
Inventors: 梶原　桂
Original assignee: 株式会社神戸製鋼所
Priority date: 2014-04-18
Filing date: 2015-04-16
Publication date: 2015-10-22
Also published as: KR101736019B1; KR20160124239A; US20170037496A1; JP2015206071A; DE112015001872T5; JP6284813B2; CN106232847B; MX2016013517A; CN106232847A

Abstract

This hot-rolled steel sheet has a thickness of 3-20 mm and contains specific amounts of C, Si, Mn, P, S, Al and N with the balance made up of iron and unavoidable impurities. The contents of solid-solved N, C and N are within specific ranges, and bainitic ferrite having a specific average crystal grain size and pearlite have specific area occupancies in the structure, with the balance occupied by polygonal ferrite. This hot-rolled steel sheet has a specific hardness distribution in the thickness direction.

Description

Hot-rolled steel sheet with excellent cold workability and excellent hardness after processing

The present invention shows a good cold workability (strong cold workability) during processing that causes extremely high deformation strain locally in cold working, and a heat that shows a predetermined hardness after working. It relates to rolled steel sheets.

In recent years, from the viewpoint of environmental protection, for the purpose of improving the fuel efficiency of automobiles, there is an increasing demand for reducing the weight of steel materials used for various parts for automobiles, for example, transmission parts such as gears and cases, that is, increasing the strength. . In order to meet such demands for weight reduction and high strength, steel materials obtained by hot forging steel bars (hot forging materials) have been used as steel materials that are generally used. In addition, in order to reduce CO ₂ emissions in the component manufacturing process, there is an increasing demand for cold forging of components such as gears that have been processed by hot forging.

By the way, cold working (cold forging) has advantages of higher productivity than hot working and warm working and good dimensional accuracy and yield of steel materials. However, the problem in manufacturing parts by such cold working is that in order to ensure the strength of the cold-worked parts to be higher than the expected value, the strength, ie deformation, is inevitably required. It is necessary to use a steel material with high resistance. However, the higher the deformation resistance of the steel material used, the shorter the service life of the cold working mold.

Also, in the field of transmission parts, from steel bar forging (hot forging, cold forging, etc.), the study of parts production using steel plates is also progressing with the aim of reducing the weight and cost of parts. However, in cold working (press forming, forging, etc.) of steel plates, transmission parts have a complicated shape, so there are some parts where extremely high deformation strain (approximately 2 or more in terms of true strain) occurs locally. There is a difficulty that exists and is easy to generate a local crack.

For this reason, conventionally, after cold forging a steel material into a predetermined shape, a method of manufacturing a high-strength part with a predetermined strength (hardness) is performed by performing a heat treatment such as quenching and tempering. There was also. However, since heat treatment after cold forging inevitably changes the part dimensions, it is necessary to correct by secondary machining such as cutting, and a solution that can omit heat treatment and subsequent machining is desired. It was rare.

In order to solve the above-mentioned problem, for example, by using solute C in low carbon steel, the progress of normal temperature aging is suppressed, and a predetermined age hardening amount due to strain aging is ensured, thereby providing a cold having excellent strain aging characteristics. It is disclosed that a wire rod and steel bar for forging can be obtained (see Patent Document 1).

However, this technology controls strain aging only by the amount of solute C, and it is difficult to obtain a steel material having sufficient cold workability and required surface quality, hardness and strength after processing. Met.

Therefore, the present applicant paid attention to the difference in the effects of solute C and solute N contained in steel materials on deformation resistance and static strain aging, and as a result of various studies, the amount of these solute elements was appropriately determined. By controlling, it is found that a steel material for machine structure showing a predetermined hardness (strength) can be obtained after cold working (cold forging) while exhibiting good cold workability during working, A patent application has already been filed (see Patent Document 2).

This steel material realizes both cold workability and high hardness (high strength) after processing, but is a hot forging material, like the wire rod and bar steel described in Patent Document 1 above. The manufacturing cost is high. Therefore, in order to further reduce the manufacturing cost, it has been studied to produce automobile parts by cold working using hot-rolled steel sheets instead of the conventional hot forging materials.

For example, a hot-rolled steel sheet for nitriding that has a high surface hardness and a sufficient hardening depth after nitriding has been proposed (see Patent Document 3).

However, this technique requires further nitriding after cold working, and there is a problem that a sufficient cost reduction cannot be realized.

Further, it contains C: 0.10% or less, Si: less than 0.01%, Mn: 1.5% or less, and Al: 0.20% or less, and (Ti + Nb) / 2: 0.05-0. 50%, S: 0.005% or less, N: 0.005% or less, O: 0.004% or less, and the total of S, N and O is 0.0100% or less A hot-rolled steel sheet having a composition and having a microstructure of 95% or more of a substantially ferritic single-phase structure has been proposed. This hot-rolled steel sheet has excellent dimensional accuracy of a precision punched surface and a stamped surface after processing. Is extremely high in surface hardness and is also excellent in red scale resistance (see Patent Document 4).

However, in this hot-rolled steel sheet, N is limited to a very low content as a harmful element, and the technical idea is completely different from the hot-rolled steel sheet according to the present invention that actively uses N. It is.

Japanese Unexamined Patent Publication No. 10-306345 Japanese Unexamined Patent Publication No. 2009-228125 Japanese Unexamined Patent Publication No. 2007-162138 Japanese Unexamined Patent Publication No. 2004-137607

The present invention has been made by paying attention to the above circumstances, and its purpose is to show good cold workability (strong cold workability) during working that causes extremely high strain in cold working. It is to provide a hot-rolled steel sheet having a predetermined hardness after processing.

A hot-rolled steel sheet excellent in strong cold workability and hardness after processing according to the first invention of the present invention,
The plate thickness is 3-20mm,
Ingredient composition is mass%,
C: more than 0% and 0.3% or less,
Si: more than 0% and 0.5% or less,
Mn: 0.2 to 1%
P: more than 0% and 0.05% or less,
S: more than 0% and 0.05% or less,
Al: 0.01 to 0.1%,
N: 0.008 to 0.025%,
The balance consists of iron and inevitable impurities,
Solid solution N: 0.007% or more, and
The content of C and N satisfies the relationship of 10C + N ≦ 3.0,
The structure is an area ratio with respect to the entire structure, bainitic ferrite: 5% or more, pearlite: less than 20%, the balance: polygonal ferrite,
The average grain size of the bainitic ferrite is in the range of 3 to 50 μm;
When the hardness distribution in the thickness direction is the surface portion, t / 4 where t is the thickness, and the maximum value of the Vickers hardness at three locations in the center is Hv _max and the minimum value is Hv _min. , (Hv _max −Hv _min ) / Hv _min is 0.3 or less
It is characterized by that.

A hot-rolled steel sheet excellent in strong cold workability and hardness after processing according to the second invention of the present invention,
In the first invention,
Ingredient composition is further mass%,
Cr: more than 0% and 2% or less, and
Mo: At least one selected from the group consisting of more than 0% and 2% or less
Is included.

A hot-rolled steel sheet excellent in strong cold workability and hardness after processing according to the third invention of the present invention,
In the first or second invention,
Ingredient composition further
Ti: more than 0% and 0.2% or less,
Nb: more than 0% and 0.2% or less, and
V: At least one selected from the group consisting of more than 0% and 0.2% or less
Is included.

A hot-rolled steel sheet excellent in strong cold workability and hardness after processing according to the fourth invention of the present invention,
In any one of the first to third inventions,
Ingredient composition is further mass%,
B: Over 0% to 0.005% or less
Is included.

A hot-rolled steel sheet excellent in strong cold workability and hardness after processing according to the fifth invention of the present invention,
In any one of the first to fourth inventions,
Ingredient composition is further mass%,
Cu: more than 0% and 5% or less,
Ni: more than 0% and 5% or less, and
Co: at least one selected from the group consisting of more than 0% and not more than 5%
Is included.

A hot-rolled steel sheet excellent in strong cold workability and hardness after processing according to the sixth invention of the present invention,
In any one of the first to fifth inventions,
Ingredient composition is further mass%,
Ca: more than 0% and 0.05% or less,
REM: more than 0% and 0.05% or less,
Mg: more than 0% and 0.02% or less,
Li: more than 0% and 0.02% or less,
Pb: more than 0% and 0.5% or less, and
Bi: At least one selected from the group consisting of more than 0% and 0.5% or less
Is included.

According to the present invention, in a structure mainly composed of bainitic ferrite and polygonal ferrite having a predetermined average particle diameter, a solid solution N amount is ensured, and the C content and the N content have a predetermined relationship. By satisfying, the deformation resistance during cold working is reduced, the life of the mold is extended, and the hardness distribution in the plate thickness direction is limited within a predetermined range, thereby extremely high deformation locally. Even in cold working that causes strain, local cracks are unlikely to occur, and the parts obtained after working can provide a hot-rolled steel sheet that can ensure a predetermined post-working hardness.

In an Example, it is a figure which shows schematic structure of the wedge type | mold compression test apparatus used in order to evaluate strong cold workability.

Hereinafter, the hot-rolled steel sheet according to the present invention (hereinafter also referred to as “the steel sheet of the present invention” or simply “the steel sheet”) will be described in more detail. The steel sheet of the present invention is common to the hot forging material described in Patent Document 2 above in that the N solid solution amount is ensured and the C content and the N content satisfy a predetermined relationship. The C content is allowed to a higher range, the structure is a bainitic ferrite-polygonal ferrite-pearlite double phase structure, the bainitic ferrite grains are refined, and the hardness in the thickness direction The difference is that the distribution is limited within a predetermined range.

[Thickness of the steel sheet of the present invention: 3 to 20 mm]
First, the steel sheet of the present invention has a thickness of 3 to 20 mm. If the plate thickness is less than 3 mm, rigidity as a structure cannot be secured. On the other hand, if the plate thickness exceeds 20 mm, it is difficult to achieve the tissue form defined in the present invention, and the desired effect cannot be obtained. A preferred plate thickness is 4 to 19 mm.

Next, the component composition constituting the steel sheet of the present invention will be described. Hereinafter, all the units of chemical components are mass%.

[Component composition of the steel sheet of the present invention]
<C: more than 0% and 0.3% or less>
C is an element that has a great influence on the formation of the structure of a steel sheet, and the structure is bainitic ferrite-polygonal ferrite-pearlite multiphase structure, but bainitic ferrite-polygonal ferrite mainly containing as little pearlite as possible. It is an element whose content needs to be limited in order to form a structure. When C is contained excessively, the pearlite fraction in the steel sheet structure increases, and the deformation resistance may be excessive due to work hardening of the pearlite. Therefore, the C content in the steel sheet is limited to 0.3% or less, preferably 0.25% or less, more preferably 0.2% or less, and particularly preferably 0.15% or less. However, if the content of C is too small, deoxidation during the melting of steel becomes difficult, and it becomes difficult to satisfy the strength and hardness after cold working, so 0.0005% or more, more preferably Is 0.0008% or more, particularly preferably 0.001% or more.

<Si: more than 0% and 0.5% or less>
Si is an element that needs to be reduced as much as possible in order to increase the deformation resistance of the steel sheet by dissolving in steel. Therefore, the Si content in the steel sheet is 0.5% or less, preferably 0.45% or less, more preferably 0.4% or less, and particularly preferably 0.3% or less in order to suppress an increase in deformation resistance. Restrict to. However, if the Si content is extremely small, deoxidation during melting becomes difficult, and it becomes difficult to satisfy the strength and hardness after cold working, so 0.005% or more, more preferably 0.008% or more, particularly preferably 0.01% or more.

<Mn: 0.2-1%>
Mn is an element having a deoxidizing and desulfurizing action in the steel making process. Furthermore, when the content of N in the steel material is increased, cracks are likely to occur due to dynamic strain aging due to heat generation during processing. On the other hand, Mn improves the workability at that time and has the effect of suppressing cracks. . In order to effectively exhibit these actions, the Mn content in the steel material is 0.2% or more, preferably 0.22% or more, and more preferably 0.25% or more. However, when the Mn content is excessive, the deformation resistance becomes excessive and the structure is not uniform due to segregation. Therefore, the content is set to 1% or less, preferably 0.98% or less, and more preferably 0.95% or less.

<P: more than 0% and 0.05% or less>
P is an impurity element inevitably contained in the steel, but if it is contained in ferrite, it segregates at the ferrite grain boundaries and degrades the cold workability. It is an element that causes an increase. Therefore, it is desirable to reduce the P content as much as possible from the viewpoint of cold workability. However, since extreme reduction leads to an increase in steelmaking costs, it is 0.05% or less, preferably in consideration of process capability. 0.03% or less.

<S: more than 0% and 0.05% or less>
S, like P, is an unavoidable impurity and is an element that precipitates in the form of a film at the grain boundary as FeS and degrades workability. It also has the effect of causing hot brittleness. Therefore, from the viewpoint of improving the deformability, in the present invention, the S content is set to 0.05% or less, preferably 0.03% or less. However, it is industrially difficult to reduce the S content to zero. In addition, since S has an effect of improving machinability, it is recommended to contain 0.002% or more, more preferably 0.006% or more from the viewpoint of improving machinability.

<Al: 0.01 to 0.1%>
Al is an element effective for deoxidation in the steelmaking process. In order to obtain this deoxidation effect, the Al content in the steel material is set to 0.01% or more, preferably 0.015% or more, and more preferably 0.02% or more. However, if the Al content is excessive, the toughness is reduced and cracking is likely to occur. Therefore, the content is 0.1% or less, preferably 0.09% or less, and more preferably 0.08% by mass or less.

<N: 0.008 to 0.025%>
N is an important element for obtaining a predetermined strength by static strain aging after processing. Therefore, the N content in the steel material is 0.008% or more, preferably 0.0085% or more, and more preferably 0.009% or more. However, if the N content is excessive, in addition to static strain aging, the influence of dynamic strain aging during processing becomes significant, and deformation resistance increases, which is unsuitable. Therefore, 0.025% or less, preferably 0 0.02% or less, more preferably 0.02% or less.

<Solution N: 0.007% or more>
By securing a predetermined amount of solid solution N in the steel sheet (hereinafter referred to as “solid solution N amount”), the static strain aging can be promoted without significantly increasing the deformation resistance. In order to ensure the required strength after cold working, the amount of solute N needs to be 0.007% or more. However, when the amount of solute N becomes excessive, cold workability deteriorates and the amount of solute N fixed to the processing strain increases, and hardness distribution tends to occur in the thickness direction of the hot-rolled sheet. Thus, even if the annealing conditions described later are applied, the hardness distribution in the plate thickness direction cannot be eliminated, and cracking is likely to occur due to processing that causes extremely high deformation strain locally. For this reason, it is preferably 0.03% or less. In addition, since content of N in steel materials is 0.025% or less, the amount of solute N does not become 0.025% or more substantially.

Here, the solid solution N amount in the present invention is an amount obtained by subtracting the amount of all N compounds from the total N amount in the steel material in accordance with JIS G 1228. A practical method for measuring the amount of dissolved N will be exemplified below.

(A) Inert gas melting method-thermal conductivity method (measurement of total N content)
A sample cut from the test material is put in a crucible, extracted in an inert gas stream to extract N, the extract is transported to a thermal conductivity cell, and the change in thermal conductivity is measured to determine the total N amount. Ask.
(B) Ammonia distillation separation indophenol blue spectrophotometry (measurement of total N compound amount)
A sample cut out from the test material is dissolved in a 10% AA-based electrolytic solution, subjected to constant current electrolysis, and the total amount of N compounds in the steel is measured. The 10% AA electrolyte used is a non-aqueous solvent electrolyte consisting of 10% acetone, 10% tetramethylammonium chloride, and the remainder methanol, and does not generate a passive film on the steel surface.

About 0.5 g of the sample material is dissolved in this 10% AA-based electrolyte, and the resulting insoluble residue (N compound) is filtered through a polycarbonate filter having a hole size of 0.1 μm. The obtained insoluble residue is decomposed by heating in a chip made of sulfuric acid, potassium sulfate and pure copper, and the decomposition product is combined with the filtrate. After making this solution alkaline with sodium hydroxide, steam distillation is performed, and the distilled ammonia is absorbed in dilute sulfuric acid. Further, phenol, sodium hypochlorite and sodium pentacyanonitrosyl iron (III) are added to form a blue complex, and the absorbance is measured using an absorptiometer to determine the total N compound amount.
And the amount of solid solution N can be calculated | required by subtracting the total N compound amount calculated | required by the method of said (b) from the total N amount calculated | required by the method of said (a).

<The content of C and N satisfies the relationship of 10C + N ≦ 3.0>
In the steel material of the present invention, solid solution C greatly increases deformation resistance and does not contribute much to static strain aging, while solid solution N promotes static strain aging without significantly increasing deformation resistance. Therefore, the hardness after processing can be increased. Therefore, in the steel material of the present invention, in order to increase the hardness after processing without significantly increasing the deformation resistance during processing, the content of C and the content of N are 10C + N ≦ 3.0. It is essential to satisfy the relationship, preferably 0.009 ≦ 10C + N ≦ 2.8, more preferably 0.01 ≦ 10C + N ≦ 2.5, and particularly preferably 0.01 ≦ 10C + N ≦ 2.0. From the viewpoint of refining crystal grains in a hot-rolled steel sheet and ensuring the formability of the steel sheet, a C content and a solute C content are required to some extent, but when 10C + N> 3.0, C and / or N The amount becomes excessive and the deformation resistance becomes excessive. Here, in the above inequality, the coefficient of the C content is set to 10 times the coefficient of the N content because the solid solution C has the same content as the solid solution N but the strength in the hot-rolled steel sheet of the present invention. Further, it is considered that the degree of increasing the deformation resistance is about one digit (10 times) larger.

The steel of the present invention basically contains the above components, and the balance is iron and inevitable impurities, but the following permissible components can be added as long as the effects of the present invention are not impaired.

<Cr: more than 0% and 2% or less, and
Mo: at least one selected from the group consisting of more than 0% and 2% or less>
Cr is an element that has the effect of improving the deformability of steel by increasing the strength of the grain boundaries. In order to effectively exhibit such an effect, Cr is preferably contained in an amount of 0.2% or more. . However, if Cr is excessively contained, deformation resistance increases and cold workability may be lowered. Therefore, the content is recommended to be 2% or less, more preferably 1.5% or less, especially 1% or less. Is done.

Mo is an element having an action of increasing the hardness and deformability of the steel material after processing. In order to effectively exhibit such action, Mo is 0.04% or more, more preferably 0. It is preferable to contain 0.08% or more. However, if Mo is excessively contained, the cold workability may be deteriorated. Therefore, the content is recommended to be 2% or less, further 1.5% or less, particularly 1% or less.

<Ti: more than 0% and 0.2% or less,
Nb: more than 0% and 0.2% or less, and
V: at least one selected from the group consisting of more than 0% and 0.2% or less>
These elements have a strong affinity for N, coexist with N to form N compounds, refine steel grains, improve the toughness of processed products obtained after cold working, and improve crack resistance. It is an element that has a role to improve. However, even if each element is contained exceeding the upper limit value, the effect of improving the characteristics cannot be obtained. It is recommended that the content of each element is 0.2% or less, further 0.001 to 0.15%, particularly 0.002 to 0.1%.

<B: more than 0% and 0.005% or less>
B, like Ti, Nb, and V, has a strong affinity with N, and coexists with N to form an N compound, refines the grain of steel, and improves the toughness of the workpiece obtained after cold working And an element having a role of improving crack resistance. Therefore, when the steel sheet of the present invention contains B, the required solid solution N amount can be secured and the strength after cold working can be improved, so the content is 0.005% or less, and further 0 0.0001 to 0.0035%, especially 0.0002 to 0.002% is recommended.

<Cu: more than 0% and 5% or less,
Ni: more than 0% and 5% or less, and
Co: at least one selected from the group consisting of more than 0% and 5% or less>
All of these elements have the effect of strain aging and hardening the steel material, and are effective in improving the strength after processing. In order to effectively exhibit such an action, these elements are preferably contained in an amount of 0.1% or more, and more preferably 0.3% or more. However, if the content of these elements is excessive, the effects of strain aging and hardening of the steel material, and the effect of improving the strength after processing are saturated, and there is a possibility of promoting cracking. In the following, 4% or less, particularly 3% or less is recommended.

<Ca: 0.05% or less (excluding 0%),
REM: 0.05% or less (excluding 0%),
Mg: 0.02% or less (excluding 0%),
Li: 0.02% or less (excluding 0%),
Pb: 0.5% or less (excluding 0%), and
Bi: at least one selected from the group consisting of 0.5% or less (excluding 0%)>
Ca is an element that spheroidizes sulfide compound inclusions such as MnS, improves the deformability of steel, and contributes to improvement of machinability. In order to effectively exhibit such an action, Ca is preferably contained in an amount of 0.0005% or more, more preferably 0.001% or more. However, even if contained excessively, the effect is saturated and an effect commensurate with the content cannot be expected. Therefore, 0.05% or less, further 0.03% or less, particularly 0.01% or less is recommended.

REM is an element that, like Ca, spheroidizes compound inclusions such as MnS to increase the deformability of steel and contribute to the improvement of machinability. In order to effectively exhibit such an action, REM is preferably contained in an amount of 0.0005% or more, more preferably 0.001% or more. However, even if contained excessively, the effect is saturated and an effect commensurate with the content cannot be expected. Therefore, 0.05% or less, further 0.03% or less, particularly 0.01% or less is recommended.
In the present invention, REM means a lanthanoid element (15 elements from La to Ln), Sc (scandium) and Y (yttrium). Among these elements, it is preferable to contain at least one element selected from the group consisting of La, Ce and Y, more preferably La and / or Ce.

Mg, like Ca, is an element that spheroidizes sulfide compound inclusions such as MnS to enhance the deformability of steel and contribute to the improvement of machinability. In order to effectively exhibit such an action, Mg is preferably contained in an amount of 0.0002% or more, more preferably 0.0005% or more. However, even if contained excessively, the effect is saturated and an effect commensurate with the content cannot be expected, so 0.02% or less, further 0.015% or less, and particularly 0.01% or less is recommended.

Li can spheroidize sulfide compound inclusions such as MnS and increase the deformability of steel like Ca, and lower the melting point of Al-based oxides to make them harmless and improve machinability. It is a contributing element. In order to effectively exhibit such an action, Li is preferably contained in an amount of 0.0002% or more, and more preferably 0.0005% or more. However, even if contained excessively, the effect is saturated and an effect commensurate with the content cannot be expected, so 0.02% or less, further 0.015% or less, and particularly 0.01% or less is recommended.

Pb is an effective element for improving machinability. In order to effectively exhibit such an action, Pb is preferably contained in an amount of 0.005% or more, and more preferably 0.01% or more. However, if it is contained excessively, production problems such as generation of rolling defects occur, so 0.5% or less, further 0.4% or less, particularly 0.3% or less is recommended.

Bi is an element effective for improving the machinability like Pb. In order to effectively exhibit such an action, Bi is preferably contained in an amount of 0.005% or more, and more preferably 0.01% or more. However, since the effect of improving the machinability is saturated even if contained excessively, 0.5% by mass or less, further 0.4% or less, particularly 0.3% or less is recommended.

Next, the structure characterizing the steel sheet of the present invention will be described.

[Structure of the steel sheet of the present invention]
As described above, the steel sheet according to the present invention is based on bainitic ferrite-polygonal ferrite pearlite double phase steel, and in particular, controls the bainitic ferrite grain size within a specific range, The hardness distribution in the thickness direction is controlled.

<Bainitic ferrite: 5% or more, pearlite: less than 20%, balance: polygonal ferrite>
The structure of the steel sheet of the present invention is composed of a multiphase structure of bainitic ferrite, polygonal ferrite and pearlite. Bainitic ferrite has the effect of improving the workability during cold working and increasing the hardness after processing while suppressing the occurrence of stretcher strain marks. The area ratio is 5% or more, preferably 10% or more, and more preferably 15% or more. The upper limit of the area ratio of bainitic ferrite in the steel sheet of the present invention is substantially about 90%, preferably 85%, more preferably 80%. Moreover, since the formability of the steel sheet is deteriorated when excessive pearlite is present, the pearlite is 20% or less in area ratio, more preferably 19% or less, still more preferably 18% or less, and particularly preferably 15% or less. The lower limit of the area ratio of pearlite in the steel sheet of the present invention is substantially about 0.5%, preferably 1%. The balance is polygonal ferrite, but the area ratio of polygonal ferrite is preferably 5% or more.
In addition to the above structure, a cementite phase is also present in the structure of the steel sheet of the present invention, but the area ratio is at most 1% or less, so in this specification, The area ratios of tick ferrite, polygonal ferrite, and pearlite were defined as those normalized so that the total area ratio of these three phases was 100%.

<Average crystal grain size of the bainitic ferrite: in the range of 3 to 50 μm>
The average grain size of bainitic ferrite constituting the bainitic ferrite structure needs to be in the range of 3 to 50 μm in order to improve the workability of the steel sheet and satisfy the surface properties after processing. . If the bainitic ferrite grains become too fine, the deformation resistance becomes too high, so the average crystal grain size is 3 μm or more, preferably 4 μm or more, more preferably 5 μm or more. On the other hand, if the bainitic ferrite is too coarse, the surface properties after processing deteriorate, and the toughness and fatigue characteristics deteriorate, so the average crystal grain size is 50 μm or less, preferably 45 μm or less, more preferably 40 μm. The following.

<Hardness distribution in the plate thickness direction: When the maximum value is Hv _max and the minimum value is Hv _min among the Vickers hardness at the surface portion, the plate thickness t / 4 portion, and the central portion, (Hv _max -Hv _min ) / Hv _min limited to 0.3 or less>
Since transmission parts have complex shapes, there is a region where deformation strain is extremely high locally (corresponding to about 2 or more in terms of true strain ε) during press forming and forging. In a steel plate having a large distribution (strength distribution, stress distribution), non-uniform plastic deformation occurs. In the low working area, that is, in the low deformation strain area (ε is less than about 2), the effect is small and no problem occurs. However, in the high strain area (ε is about 2 or more), local cracking occurs. End up. In order to prevent local cracks from occurring even in such an extremely high strain region where ε is about 2 or more, the hardness distribution in the thickness direction is the surface portion, the thickness t / 4 portion, and the central portion at three locations. Of the Vickers hardness, when the maximum value is HV _max and the minimum value is HV _min , (Hv _max -Hv _min ) / Hv _min is 0.3 or less, preferably 0.2 or less, more preferably 0.15 or less. Limit to.

Here, it is assumed as follows about the mechanism in which hardness distribution generate | occur | produces in a plate | board thickness direction with the conventional hot-rolled steel plate. That is, in the hot-rolled steel sheet having a large thickness, the reason why the hardness distribution occurs in the thickness direction is that the difference in the processing degree between the surface portion and the center portion inevitably generated in the hot rolling process, the surface portion and the center There is a difference in the processing temperature of the part (including processing heat generation), and further, the phase transformation during the coil cooling process, the occurrence of residual stress, etc. are also affected. In addition, since the alloy component of the present invention contains a large amount of solute N, it also affects that the hardness of such a region with a large processing strain increases due to the fixing action of N to the region with a large processing strain. . As described above, a hardness distribution in the thickness direction is generated due to a plurality of complicated factors, and variations in strength tend to occur in the thickness direction.
Therefore, the steel sheet of the present invention can be obtained by reducing the hardness distribution in the plate thickness direction by subjecting the hot rolled plate to batch annealing under the recommended conditions described later.

[Measurement method of area ratio of each phase]
Regarding the area ratio of each of the above phases, each test steel sheet was subjected to Nital corrosion, taken with a scanning electron microscope (SEM; magnification 1000 times), five fields of view, and the ratios of bainitic ferrite, polygonal ferrite and pearlite were pointed out. It can be obtained by arithmetic.
Here, bainitic ferrite is a ferrite grain that exists in the structure of bainite (generically referred to as upper bainite and lower bainite). Definition ”-understanding the current situation-heat treatment volume 50 No. 1, February 2010, p. 22-27), defined as having an aspect ratio (major axis / minor axis ratio) of 2 or more . Polygonal ferrite is defined as ferrite grains having equiaxed crystal grains and an aspect ratio (major axis / minor axis ratio) of less than 2.

[Measurement method of average crystal grain size]
The average crystal grain size of the bainitic ferrite can be measured as follows. That is, the crystal grain size of bainitic ferrite existing at three locations, the outermost layer portion, the plate thickness ¼ portion, and the plate thickness center portion, is measured. As for the particle size of one bainitic ferrite particle, the side part in the rolling direction of each measurement point was subjected to nital corrosion, and the corresponding part was photographed with five fields of view with a scanning electron microscope (SEM; magnification 1000 times). The average crystal grain size of ferrite crystal grains was determined based on the center of gravity diameter by image analysis.

[Measurement method of hardness distribution in the thickness direction]
Micro Vickers hardness test at each of the surface portion (position 400 μm deep from the plate surface), the plate thickness ¼ portion, and the plate thickness center portion in the cross section in the plate thickness direction parallel to the rolling direction of the hot-rolled steel plate. Using a machine, the Vickers hardness (Hv) was measured under the conditions of load: 50 g, number of measurements: 5 times, and the average of each was taken as Vickers hardness at each location.
Then, the maximum value Hv _max and the minimum value Hv _min were obtained from the Vickers hardness at these three locations, and (Hv _max −Hv _min ) / Hv _min was calculated.

Next, a preferred manufacturing method for obtaining the steel sheet of the present invention will be described below.

[Preferred production method of the steel sheet of the present invention]
The steel plate of the present invention may be produced according to any method as long as the raw steel having the above composition can be formed into a desired plate thickness. For example, it can be carried out by preparing a molten steel having the above component composition in a converter under the conditions shown below, slab this by ingot casting or continuous casting, and then rolling it into a hot-rolled steel sheet having a desired thickness. .

[Preparation of molten steel]
The N content in the molten steel is adjusted by adding a raw material containing an N compound to the molten steel and / or controlling the converter atmosphere to an N ₂ atmosphere during melting in the converter. can do.

[heating]
Heating before hot rolling is performed at 1100 to 1300 ° C. This heating requires high-temperature heating conditions in order to dissolve as much N as possible without producing an N compound. The minimum with a preferable heating temperature is 1100 degreeC, and a more preferable minimum is 1150 degreeC. On the other hand, temperatures exceeding 1300 ° C. are difficult to operate.

[Hot rolling]
Hot rolling is performed so that the finish rolling temperature is 880 ° C. or higher. If the finish rolling temperature is too low, ferrite transformation will occur at a high temperature, and the precipitated carbides in ferrite (generically referred to as bainitic ferrite and polygonal ferrite) will become coarse and fatigue strength will deteriorate. The above finish rolling temperature is required. The finish rolling temperature is more preferably 900 ° C. or higher in order to coarsen the austenite grains and increase the grain size of the bainitic ferrite to some extent. The upper limit of the finish rolling temperature is set to 1000 ° C. because it is difficult to secure the temperature.

The thickness of the hot-rolled steel sheet of the present invention is 3 to 20 mm. In order to refine the bainitic ferrite crystal grains and control the average crystal grain size within a predetermined grain size range, In addition to control, it is necessary to set the final reduction ratio of tandem rolling of finish rolling to 15% or more. Normally, the finish rolling is performed by tandem rolling of 5 to 7 passes, but a pass schedule is set from the viewpoint of control of sheet penetration, and the final rolling reduction is about 12 to 13%. The final rolling reduction is preferably 16% or more, more preferably 17% or more. The higher the final reduction ratio is 20% or 30%, the more effective the crystal grains are refined, but the upper limit is defined to be about 30% from the viewpoint of rolling control.

[Rapid cooling after hot rolling]
After finishing the finish rolling, quenching is performed at a cooling rate (first quenching rate) of 20 ° C./s or more within 5 s, and rapid cooling is stopped at a temperature of 550 ° C. or more and less than 650 ° C. (quenching stop temperature). This is for obtaining a bainitic ferrite-polygonal ferrite-pearlite double phase structure having a predetermined phase fraction. When the cooling rate (quenching rate) is less than 20 ° C./s, the pearlite transformation is promoted, or when the quenching stop temperature is less than 550 ° C., the bainite transformation is suppressed, both of which are bainitic ferrite-polygonal having a predetermined phase fraction. It becomes difficult to obtain ferrite-pearlite steel, and cold workability and surface quality after processing deteriorate. On the other hand, when the quenching stop temperature is 650 ° C. or more, the precipitated carbide in the ferrite is coarsened, and the fatigue strength is deteriorated. The quenching stop temperature is preferably 560 to 640 ° C, more preferably 580 to 620 ° C.

[Slow cooling after rapid cooling stop]
After the rapid cooling is stopped, it is slowly cooled for 5 to 20 seconds at a cooling rate (slow cooling rate) of 10 ° C./s or less by cooling or air cooling. Thereby, the formation of polygonal ferrite is sufficiently advanced, and the precipitated carbide in the ferrite is appropriately refined. When the cooling rate exceeds 10 ° C./s or the slow cooling time is less than 5 s, the amount of polygonal ferrite formed is insufficient. On the other hand, if the slow cooling time exceeds 20 s, the precipitated carbide is not coarsened and the fatigue strength is deteriorated.

[Rapid cooling after slow cooling, winding]
After the slow cooling, it is rapidly cooled again at a cooling rate (second quenching rate) of 20 ° C./s or more, and wound at 500 to 600 ° C. This is because cold workability is ensured by using a bainitic ferrite + polygonal ferrite-based structure. When the cooling rate (second rapid cooling rate) is less than 20 ° C./s or the coiling temperature exceeds 600 ° C., a large amount of pearlite is formed and the cold workability is deteriorated. As a result, the surface quality after processing is deteriorated.

[Batch annealing after hot rolling]
After the hot rolling, in order to limit the hardness distribution in the thickness direction within the above predetermined range, the hot rolled plate (hot rolled coil) is subjected to batch annealing under the following conditions.
That is, in this batch annealing, in order to suppress generation of surface scale and decarburization, the steel sheet is heated from room temperature to 400 ° C. or higher and Ac1 or lower in an atmosphere of H ₂ : 15 to 20% by volume, and then 1 h or longer and 15 h or shorter. Hold and do.
The holding temperature and holding time vary depending on the thickness of the hot rolled plate and the size of the coil, but the degree of hardness distribution limit required in correspondence with the required cold working degree, It is appropriately selected depending on the uniformity of the temperature inside the coil.
This heat treatment removes residual stress generated during hot rolling, softens it, reduces strain, promotes the release of fixed N elements and spheroidization of carbides, and dissolves fine lamellae in austenite. By doing so, the hardness distribution in the plate thickness direction is reduced. After the batch annealing, the steel sheet is cooled to 600 ° C. at a rate of 10 ° C./h or less, thereby promoting the spheroidization of the carbide. Next, the cooling is performed at a rate of 15 ° C./h or less from 600 to 400 ° C., in order to stabilize the shape such as coil crushing by uniformly cooling the inside of the coil. Thereafter, at a temperature of 400 ° C. or lower, cooling can be performed at a high cooling rate (such as about 50 to 100 ° C./h or higher) by water cooling or the like as long as the temperature distribution in the coil can be uniformly cooled.
When the holding temperature of batch annealing is less than 400 ° C., the above effect is small. On the other hand, when the temperature exceeds the Ac1 point, the structure changes. The holding temperature is more preferably 450 to 650 ° C, particularly preferably 500 to 600 ° C.
If the holding time is less than 1 hour, the above effect is small. On the other hand, if the holding time exceeds 15 hours, the effect is saturated, the productivity is hindered, and a surface scale is easily generated, which is not preferable. The holding time is more preferably 2 to 14 h, particularly preferably 3 to 12 h.

Hereinafter, the present invention will be described in more detail with reference to examples. However, the following examples are not intended to limit the present invention, and may be implemented with appropriate modifications within a range that can meet the purpose described above and below. These are all possible and are within the scope of the present invention.

Steel having the component composition shown in Table 1 below is melted by vacuum melting, cast into an ingot having a thickness of 120 mm, and further subjected to batch annealing after hot rolling under the conditions shown in Table 2 and Table 3 below. A hot-rolled steel sheet was prepared. In any test, the cooling rate until the quenching stop after finishing rolling is 20 ° C./s or more, and the cooling after the quenching stop is a condition of slow cooling for 5 to 20 s at a cooling rate of 10 ° C./s or less. Further, after the batch annealing, cooling was performed at a cooling rate of 10 ° C./h or less up to 600 ° C., 15 ° C./h or less from 600 to 400 ° C., and water cooling was performed at 400 ° C. or less.

About the hot-rolled steel sheet thus obtained, the amount of solute N, the area ratio of each phase of the structure in the steel sheet, the average crystal grain size of bainitic ferrite, and the hardness distribution in the thickness direction are It was calculated | required by each measuring method demonstrated in the form for inventing.

Further, the hot-rolled steel sheet was evaluated for the cold workability and the hardness after working as follows.

(Evaluation of strong cold workability)
In order to evaluate the workability in cold working (strong cold workability) that causes extremely high deformation strain locally, the amount of work strain introduced into the surface of the test piece is 4 or more in terms of true strain. As shown in FIG. 1, a wedge-type compression test (with a compression speed of 1 mm / second) was performed using a cylindrical test piece and a wedge-shaped jig as shown in FIG. , 80% reduction of the test piece diameter). In addition, as a test piece, what was cut out in the column shape from the said hot-rolled steel plate so that it may become a diameter 10mm when a plate | board thickness is 10 mm or more, and a plate | board thickness is a diameter when a plate | board thickness is less than 10 mm is used. It was.

Prior to the main compression test, forging analysis software: FORGE (manufactured by TRANSVALOR) was used to calculate the distribution of the true strain in the test piece at 80% reduction in the compression test. It has been confirmed that the true strain ε is 4 or more at a position of 100 μm in depth from the surface of the portion compressed by the R portion of the compression jig in the surface portion.

And by observing the test piece after the said wedge-type compression test visually, strong cold work property was evaluated with the following evaluation criteria, and the case of (circle) was set as the pass.
○: No crack occurred in the test piece
Δ: Microcracking occurred on the surface of the test piece
×: Crack occurred in the test piece

(Evaluation of hardness after processing)
In addition, as an evaluation of the hardness after processing, the surface central portion of the portion compressed by the compression jig of the test piece after the wedge-type compression test was loaded using a Vickers hardness tester: 500 g, the number of measurements. : Vickers hardness (Hv) was measured under the conditions of 5 times, and the average was taken as the post-processing hardness, and a value of 250 Hv or higher was accepted.

These measurement results are shown in Tables 4 to 6 below.

As shown in Tables 4-6, steel no. 1-2 to 1-6, 2, 3, 7 to 14, and 25 to 28 were all manufactured using the steel types satisfying the requirements of the component composition provisions of the present invention under the recommended production conditions. Invented steel that meets the requirements of the organization regulations. Both cold workability and post-working hardness satisfy the acceptance criteria, and good strong cold during machining that causes extremely high strain in cold working. It was confirmed that a hot-rolled steel sheet having a predetermined hardness (strength) was obtained after the processing while exhibiting the workability.

In contrast, Steel No. 1-1, 1-7 to 1-10, 4 to 6, 15 to 24, and 29 are comparative steels that do not satisfy at least one of the component composition and the structure requirements defined in the present invention, and are strongly cold worked. At least one of the properties and post-processing hardness does not satisfy the acceptance criteria.

For example, steel No. 1-1 satisfies the requirements of the component composition, but is not subjected to batch annealing after hot rolling, the hardness distribution in the plate thickness direction is expanded, and at least the cold workability is inferior.

Steel No. 1-7 satisfies the requirements of the component composition, but the holding temperature of batch annealing after hot rolling is too low outside the recommended range, the hardness distribution in the plate thickness direction is expanded, and at least strong cold workability Is inferior.

On the other hand, steel No. In No. 1-8, although the requirements of the component composition are satisfied, the holding temperature of batch annealing after hot rolling is too high outside the recommended range, and the post-processing hardness is inferior.

Steel No. 1-9 satisfies the requirements for the component composition, but the holding time of batch annealing after hot rolling is too long outside the recommended range, and the post-processing hardness is inferior.

On the other hand, steel No. 1-10 satisfies the requirements of the component composition, but the holding time of batch annealing after hot rolling is too short outside the recommended range, the hardness distribution in the sheet thickness direction is expanded, and at least strong cold workability Is inferior.

Steel No. No. 4, although the requirements for the component composition are satisfied, the heating temperature before hot rolling is too low outside the recommended range, the amount of solute N is insufficient, and the hardness after processing is inferior.

Steel No. No. 5, although the requirements of the component composition are satisfied, the plate thickness after hot rolling is too large outside the specified range, the bainitic ferrite is insufficient, while it becomes coarse and the hardness after processing is inferior.

Steel No. No. 6, although the requirements for the component composition are satisfied, the final rolling reduction during hot rolling is too small outside the recommended range, and bainitic ferrite is insufficient while coarsening, resulting in poor hardness after processing.

Steel No. Although 15 (steel type j) is in the recommended range of manufacturing conditions, the N content is too low and the post-working hardness is poor.

On the other hand, steel No. No. 16 (steel type k), although the production conditions are in the recommended range, the N content is too high and at least the strong cold workability is inferior.

Steel No. 17 (steel grade l), although the production conditions are in the recommended range, the C content is too high and does not satisfy the requirement of 10C + N ≦ 3.0, pearlite is excessively formed, and at least the cold workability is poor. Yes.

Steel No. No. 18 (steel type m) has a manufacturing condition in the recommended range, but the Si content is too high, and at least the cold workability is inferior.

Steel No. Although 19 (steel type n) has a manufacturing condition in the recommended range, the Mn content is too low and the hardness after processing is inferior.

On the other hand, steel no. Although 20 (steel type o) has a production condition in the recommended range, the Mn content is too high and at least the strong cold workability is inferior.

Steel No. Although 21 (steel type p) has production conditions in the recommended range, the P content is too high and at least the strong cold workability is inferior.

Steel No. Although 22 (steel type q) has a production condition in the recommended range, the S content is too high and at least the strong cold workability is inferior.

Steel No. Although 23 (steel type r) has a production condition in the recommended range, the Al content is too low and at least the strong cold workability is inferior.

On the other hand, steel No. For 24 (steel type s), although the production conditions other than the final rolling reduction during hot rolling are in the recommended range, the Al content is too high and at least the cold workability is poor.

On the other hand, steel No. Although 29 (steel type x) is in the recommended range of manufacturing conditions, it does not satisfy the requirement of 10C + N ≦ 3.0, and at least strong cold workability is inferior.

From the above, the applicability of the present invention was confirmed.

Although the present invention has been described in detail and with reference to specific embodiments, it will be apparent to those skilled in the art that various changes and modifications can be made without departing from the spirit and scope of the invention.
This application is based on a Japanese patent application filed on April 18, 2014 (Japanese Patent Application No. 2014-086747), the contents of which are incorporated herein by reference.

The hot-rolled steel sheet of the present invention exhibits good workability during cold working and exhibits a predetermined hardness after working, and is particularly used for various parts for automobiles, such as transmission parts such as gears and cases. Useful as steel.

Claims

The plate thickness is 3-20mm,
Ingredient composition is mass%,
C: more than 0% and 0.3% or less,
Si: more than 0% and 0.5% or less,
Mn: 0.2 to 1%
P: more than 0% and 0.05% or less,
S: more than 0% and 0.05% or less,
Al: 0.01 to 0.1%,
N: 0.008 to 0.025%,
The balance consists of iron and inevitable impurities,
Solid solution N: 0.007% or more, and
The content of C and N satisfies the relationship of 10C + N ≦ 3.0,
The structure is an area ratio with respect to the entire structure, bainitic ferrite: 5% or more, pearlite: less than 20%, the balance: polygonal ferrite,
The average grain size of the bainitic ferrite is in the range of 3 to 50 μm;
When the hardness distribution in the thickness direction is the surface portion, t / 4 where t is the thickness, and the maximum value of the Vickers hardness at three locations in the center is Hv max and the minimum value is Hv min. , (Hv max −Hv min ) / Hv min is 0.3 or less
A hot-rolled steel sheet excellent in strong cold workability and surface hardness after processing.
The hot rolled steel sheet according to claim 1, wherein the component composition further contains at least one of the following (a) to (e).
(A) at least one selected from the group consisting of Cr: more than 0% and 2% and Mo: more than 0% and 2% in mass%
(B) at least one selected from the group consisting of Ti: more than 0% and 0.2% or less, Nb: more than 0% and 0.2% or less, and V: more than 0% and 0.2% or less.
(C) By mass%, B: more than 0% and 0.005% or less
(D) By mass%, at least one selected from the group consisting of Cu: more than 0% and 5% or less, Ni: more than 0% and 5% and Co: more than 0% and 5% or less (e) , Ca: more than 0% and less than 0.05%, REM: more than 0% and less than 0.05%, Mg: more than 0% and less than 0.02%, Li: more than 0% and less than 0.02%, Pb: more than 0% 0.5% or less and Bi: at least one selected from the group consisting of more than 0% and 0.5% or less